The invention relates to a surge arrester having a cage design, as is known by way of example from JP 62-149511 (application number). Surge arresters are connected between live cables and earth in electrical power supply systems in order, when an overvoltage occurs on the line, to dissipate this overvoltage to earth and thus to protect other components in the power supply network. A surge arrester such as this contains a stack of varistor blocks, which is held between two connecting elements. This arrangement is accommodated in a housing.
In order to ensure that the varistor blocks make good contact with one another even when mechanical loads are applied, the stack must be held together under pressure. In the case of surge arresters such as these with a cage design, this is achieved by means of reinforcing elements, generally rods or cables, preferably glass-fibre-reinforced plastic rods (GFC rods) which are held in tension at the two end fittings.
One problem with surge arresters such as these is to attach the reinforcing elements securely to the end fittings such that the necessary strength is maintained even in the event of mechanical loads such as those, which occur in the case of surge arresters installed in the open air.
In the cited Japanese patent application, this problem is solved by providing grooves in the stacking direction of the varistor blocks in the end fittings, into which grooves the reinforcing elements are inserted, and in which the end of the reinforcing elements is equipped with a thread onto which a nut is screwed whose diameter is larger than the groove in the end fitting, thus holding the reinforcing element—essentially by means of an interlocking connection.
Although this allows a surge arrester to be designed in an effective manner, this results in the problem of the thread cutting into the GFC rods that are used as reinforcing elements, without damaging them. This is complex and expensive.
Further options are known from European Patent Application EP 93 915 343.3, as to how reinforcing elements can be anchored on the end fittings of a surge arrester. In particular, this document proposes that the reinforcing elements be held firmly by means of a pin or a screw, which extends at right angles to the longitudinal direction of the reinforcing elements and is passed through an unthreaded hole through the rods. The pin and the screw are then held in an appropriate recess or a threaded hole in the end fitting.
Although it is considerably simpler to form a hole in the direction at right angles to the extension direction of the GFC rods which are used as reinforcing elements than to cut a thread in them, this configuration results in the risk of the reinforcing elements being weakened in the area of the hole in such a way that they tear. The cited European patent application furthermore also discloses the option of fixing the reinforcing elements in the end fitting by means of wedges. For this purpose, a wedge which tapers in the direction of the stack centre of the varistor blocks is positioned between each reinforcing element and a correspondingly inclined surface of the end fitting, and the two are held together with radial pressure by an outer part of the end fitting. If a tensile load is applied to the reinforcing elements, static friction results in the wedges being drawn together, thus ensuring that the reinforcing elements are held with a friction lock or force fit between the associated wedge and the end fitting.
In addition, DE 199 40939 discloses the option of holding the reinforcing elements in the end fitting by inserting into a conical hole in the end fitting a sleeve which tapers conically in the direction of the stack centre of the varistor blocks and has moving side walls and which sleeve, as a clamping sleeve, holds the associated reinforcing element firmly under tension by means of a friction-locking connection or force-fitting connection in a similar way to the way already cited European patent application.
Finally, WO 00/55869 proposes that the reinforcing elements be provided with crimp sleeves at their ends, in this way impeding them from sliding through guide grooves into the end fitting.
In the case of all the abovementioned documents, in which the reinforcing elements are held with a friction lock or force fit in the end fitting, it is difficult to apply the necessary tensile strength. Particularly when the surge arrester is subject to a bending load, a very strong tensile force is produced on one side of the surge arrester.
It is difficult to manufacture surge arresters with a crimp sleeve since the crimp sleeve must be applied while the reinforcing element is being held under tension. The crimping process can also result in the rod being damaged without this being evident.
In the case of those surge arresters in which the reinforcing elements are held in the end fitting by means of screws, this necessitates considerable weakening of the reinforcing element in the area of the screw.